This invention relates to TEM mode strip line directional couplers for coupling energy over a broad frequency range from a primary transmission line to a secondary transmission line with high directivity and a method for making them.
In a TEM mode directional coupler, power is coupled from a primary transmission line to a secondary transmission line by bringing the center conductors of these two lines sufficiently close together for a distance of at least one-quarter of a wavelength of the center frequency to cause interaction of the electric and magnetic fields. The end of the secondary transmission line adjacent to the primary transmission line input is connected to an output designated here as the coupled output. The opposite end of the secondary line is terminated in a matched load. In operation, a known fraction of the energy flowing in the forward direction of the primary transmission line will appear at the coupled output. However, almost no energy flowing in the reverse direction in the primary transmission line will appear at the coupled output. Directivity of the coupler is the ratio in dB of the power at the coupled output, when power is transmitted in the primary transmission line forward direction, to the power at the coupled output, when the same amount of power is transmitted in the primary transmission line reverse direction.
The directional property of a TEM mode coupler results from the fact than TEM mode coupling between parallel transmission lines is contra-directional, i.e., the wave induced in one line travels in the opposite direction from the inducing wave in the other line. Any non-TEM mode coupling will degrade directivity.
For most applications, it is desirable to have high directivity so that the signal at the coupled output will be an accurate indication of only the power traveling in the forward direction in the primary transmission line.
While it has been possible in the past to construct TEM mode strip line couplers with high directivity at one particular frequency, it has been most difficult to do so over a broad range of frequencies. In the past, many attempts have been made to increase the directivity over a broad range. One of the solutions proposed is disclosed in U.S. Pat. No. 3,204,206 which issued to Harmon on Aug. 21, 1965.
As described in that patent, conductive elements in the form of posts or shims are positioned in close proximity to the ends of the coupled portion of the transmission lines where the center conductors diverge. The posts have their center lines disposed perpendicular to the plane of the center conductors and the shims are disposed so that the normal to the shim is parallel to the plane of the center conductor. One of the disadvantages of that arrangement is that very careful and critical adjustment of the elements became necessary to maximize directivity and such adjustment has to be carried out on the bench for each coupler. Further, these conductive elements required complex modification of the dielectric slabs, after the photo-edge process, which increased the expense of the coupler.